1974 johson 70hp top rpms????

[mrbassman]

I'm a new boat owner, and purchased a 1974 reinell<br />trihull. it has a 70 hp jonson. the compresson in<br />the 3 cylinders is 110, 115, 120.was wondering what the top rpms should be under full load. with only 2 people in it 500 pounds. it caps out at 4100 tops going with the wind. seems to plane high.<br />was running great and then all of a sudden when you floor it it stalls.<br />at low rpms just putts like tiger woods,lol<br />have read about this forum for along time and there are alot of good people here. could some one please lend a hand to a rookie???<br />also does any one know the fuel tank capsities<br />of this. if someone could give me the formula i <br />will calculate myself.<br /> <br /> regards mrbassman

 

[Ted S.]

Re: 1974 johson 70hp top rpms????

First you have to decide where you want to look with this problem. Generally, you need to look at 4 factors. Spark, fuel, compression and timing.Any one of these will affect hor it performs. Since you answered the compression question, I would be looking at the spark quality. Check it with a spark tester or use a screwdriver stuck in the plug connector. Have someone crank the engine while you hold the shaft of the screwdriver about 1/8" away from any available ground source, primarily the block. You should have a crisp blue snap to the block of spark. Never under any circumstances crank the engine without grounding the plug wire. You could damage the ignition system. If that is good, go to the fuel system. Check all the lines, tanks and fittings, primer bulb for cracks, looseness and kinks. Check the lines on the motor to the fuel pump and carb, and don't forget the pulse line from the crankcase. The pulse line allows the pressure/vacuum signal required to operate the fuel pump. Also check the carbs, they may need a cleaninf if its been sitting for any length of time.Cover this stuff first and you will probably find your problem. Cheers!

 

[mrbassman]

Re: 1974 johson 70hp top rpms????

Thanks so much,will give you a follow up.<br /><br /> mrbassman

 

[DJ]

Re: 1974 johson 70hp top rpms????

mrbassman,<br /><br />4100 rpm's is far too low. Should be 5500-5800-maybe a bit more.<br /><br />As far as stalling, I've found that carb. problems cause this in OMC three lungers.